LT4 1996 LT4 with Corsa Before and After HP

[Nick S]

Ok, I have a 1996 Corvette with the LT4 engine. The car has 68,433 miles and the only modification to the car is a an SLP Cold Air Induction. After much research, I decided to go ahead and purchase a Corsa cat-back exhaust. I got the exhaust yesterday and had it installed today.

Before the installation, I had the car put on a Mustang dyno at the shop. The car made 288.4 hp at 5,750 RPM and 316.4 ft/lbs of torque at 4,250 RPM. The exhaust was installed and put back on the same dyno. The car made 290.4 hp at 5,500 RPM and 318.1 ft/lbs of torque at 4,250 RPM. That is a gain of 2 hp and 1.7 ft/lbs of torque. Not real impressive. Now, I know that people are going to ask what gear the car was in when these numbers were measured. To be honest, I only watched them make the baseline pull and I don't recall what gear the car was in, and I didn't ask them. The car was, however, registering like 72 mph at a little over 5,700 RPM before the dyno shut down.

Are these numbers awful? I know that they are lower than what you would get on a DynoJet. Roughly, what might these numbers translate to if tested on a DynoJet.

I do enjoy the exhaust. It sounds a whole lot better than the stock exhaust. Actually, it now makes sound. Oh, and if anyone in Northern Virginia wants a totally stock exhuast off of an LT4 Corvette that is in pristine condition, let me know. The shop is holding on to the exhaust for a few days until I tell them what to do with it.

Nick

 

[DRTH VTR]

I think those numbers are very respectable. My 2002 Z06 had 333HP at the rear wheels on the dyno. That was less than I had hoped for, but about what is expected.

I say that they ran my manual 6 speed in 4th, and the speedometer was more in the 130MPH area. I did not see very well. But the Z06 was screaming!

 

[Nick S]

What type of dyno did you run your Z06 on?

 

[DRTH VTR]

What type of dyno did you run your Z06 on?

It was the low end Dynojet at last year's CAC CruiseFest in Kentucky. I jokingly call it Bubba's Dyno. I understand that there is 15-20% drive train power loss on the way to the rear wheels. That makes my stated 405HP about on the money at 333HP. Using the same logic, that would be about 350HP at the flywheel for you. That's about correct, isn't it? Didn't Chevy advertise 340HP for the LT4?

 

[Nick S]

Yeah, Chevy said 330 hp for the LT4. From what I have seen and read, I figured that the engine would be between 350 and 360 at the flywheel.

I guess that I was a little surprised that the Corsa did not add more than 2 hp. I had figured that it should have added maybe 10 hp. I guess that the stock exhaust must be pretty good. The sound of the Corsa is nice. It isn't super loud, but it isn't quiet either.

 

[Louis Bartay]

Yeah, Chevy said 330 hp for the LT4. From what I have seen and read, I figured that the engine would be between 350 and 360 at the flywheel.

I guess that I was a little surprised that the Corsa did not add more than 2 hp. I had figured that it should have added maybe 10 hp. I guess that the stock exhaust must be pretty good. The sound of the Corsa is nice. It isn't super loud, but it isn't quiet either.

Hang on to your stock exhaust. It is 2 3/4 inch except where the resonator is and where the rear mufflers are. GM reduced the size to 2 1/2 at those two places. If you ever have a 383 or larger crank, rods etc. installed you can cut off the resonator and replace it with a 2 3/4 X pipe and replace the rear mufflers with Magnaflows that are 2 3/4 inlet and have a 3 inch inside ( there are other mufflers that will work too) and you will have a better cat back than any 2 1/2 after market. The down side is interior noise with the resonator gone.

If you will have your OBDII computer remapped on a Dyno you will see about 10 more HP at the rear wheels. My 2 1/2 Borla cat back did not help much but it does sound Great.I need to have mine Dyno tuned too. GM built a good exhaust system for the LT4 stock.

 

[Nick S]

Well, I don't have any place to store the stock exhaust that was taken off. So, unless someone claims it today, I am having the shop that did the install toss it. If I hung on to every part that I replaced on the car, I'd probably be living in a Sanford and Son episode (a junk yard).

So, is the consensus here that the dyno numbers that I got for my car were ok for a pretty much stock LT4?

 

[Louis Bartay]

Well, I don't have any place to store the stock exhaust that was taken off. So, unless someone claims it today, I am having the shop that did the install toss it. If I hung on to every part that I replaced on the car, I'd probably be living in a Sanford and Son episode (a junk yard).

So, is the consensus here that the dyno numbers that I got for my car were ok for a pretty much stock LT4?

Yes most stock good running LT4 make about 290/310 at the rear wheels. Shame you can not keep the stock exhaust. You will feel the difference after you have your OBDII remapped for what you have done. go to GrandSport Registry.com and click on the dyno charts.

 

[tlong]

You might consider PCMFORLESS. Send in your ECM with the intake/exhaust mods and let Bryan do his thing. Mine ran stronger with better shifts and fan on at 208 off at 196 for cool running. Idles better too.

 

[JEFNLSA]

You might consider PCMFORLESS. Send in your ECM with the intake/exhaust mods and let Bryan do his thing. Mine ran stronger with better shifts and fan on at 208 off at 196 for cool running. Idles better too.

I agree...

I'm sure that he could pull a few more ponies out that baby. I sent mine to him w/ good results. Definitely the most affordable "custom tune" you can get.

 

[Nick S]

Does anyone know the Web site address for PCMFORLESS?

 

[Nick S]

I'll now answer my own question: http://www.pcmforless.com

 

[JEFNLSA]

I'll now answer my own question: http://www.pcmforless.com

Give Bryan a call...

If you need it back to you quick, you can go next or 2nd day air. As long as he's not too busy, he really only needs the PCM for a day or so...

 

[Louis Bartay]

Give Bryan a call...

If you need it back to you quick, you can go next or 2nd day air. As long as he's not too busy, he really only needs the PCM for a day or so...

Thanks I will do the same.
Louis

 

[Vis Croceus]

Typical gain for a catback is about 15-20 RWHP.

http://www.grandsportregistry.com/dyno.htm

 

[Aurora40]

Are those numbers SAE adjusted? If not, it will be very hard to make meaningful comparisons between them.

However, the stock LT1/LT5 system is resonably ok with the mufflers being the biggest restriction. From what I understand, the LT4 mufflers were freer than this. Here is a good article on the stock '93+ LT5 system (which is basically the same as the LT1 exhaust): http://www.zr1specialist.com/HAT Web/articles/backpressurea.jpg

I couldn't say how the LT4 exhaust compares to this or how much freer it is. But the Corsa system makes as big a gain on LT5's as any other systems. It may be that your dyno read is not overly comparable, or that the LT4 isn't restricted by the exhaust as much. Or maybe you just need an adjustment in tune to make use of the freer flow.

The main thing is that you enjoy the sound though. You'll notice that more often than you would an extra 10ish horsepower.

 

[Louis Bartay]

Are those numbers SAE adjusted? If not, it will be very hard to make meaningful comparisons between them.

However, the stock LT1/LT5 system is resonably ok with the mufflers being the biggest restriction. From what I understand, the LT4 mufflers were freer than this. Here is a good article on the stock '93+ LT5 system (which is basically the same as the LT1 exhaust): http://www.zr1specialist.com/HAT Web/articles/backpressurea.jpg

I couldn't say how the LT4 exhaust compares to this or how much freer it is. But the Corsa system makes as big a gain on LT5's as any other systems. It may be that your dyno read is not overly comparable, or that the LT4 isn't restricted by the exhaust as much. Or maybe you just need an adjustment in tune to make use of the freer flow.

The main thing is that you enjoy the sound though. You'll notice that more often than you would an extra 10ish horsepower.

I agree, the Sound. Is the LT5 a 2 3/4 inch dia.? If so just change the mufflers and have a bigger cat back. The Corsa is a 2 1/2 like my Borla on my LT4. I am going to sell the Borla and replace the resonator and mufflers on my stock LT4 system(2 3/4) and have a better and bigger cat back.

 

[Aurora40]

Is the LT5 a 2 3/4 inch dia.? If so just change the mufflers and have a bigger cat back.

Yes, but on the '90-92 LT5, the resontator is much worse than the LT1. My resonator tapered down to about 2" in diameter through it. The LT1/4 is around 2.5" pipes. I am sure that contributed to the 22rwhp gain as well as the muffler change.

In terms of pipe size, though, I would say it's certainly possible for a 2.5" pipe that is basically straight pipe to flow more than a 2.75-3" system with baffling, perforation, etc. All of those hurt flow. Perforation, as common with most glasspacking, actually takes a rather big toll on flow though it doesn't seem like it should. I think that is why Corsa can make power like the biggest systems from 2.5". They have basically straight pipe. I have Corsas on my Aurora and in the muffler there is just a narrow ring cut out to allow access to the chambers, for practically the complete length it is solid, not perforated, 2.5" pipe.

 

[Louis Bartay]

Yes, but on the '90-92 LT5, the resontator is much worse than the LT1. My resonator tapered down to about 2" in diameter through it. The LT1/4 is around 2.5" pipes. I am sure that contributed to the 22rwhp gain as well as the muffler change.

In terms of pipe size, though, I would say it's certainly possible for a 2.5" pipe that is basically straight pipe to flow more than a 2.75-3" system with baffling, perforation, etc. All of those hurt flow. Perforation, as common with most glasspacking, actually takes a rather big toll on flow though it doesn't seem like it should. I think that is why Corsa can make power like the biggest systems from 2.5". They have basically straight pipe. I have Corsas on my Aurora and in the muffler there is just a narrow ring cut out to allow access to the chambers, for practically the complete length it is solid, not perforated, 2.5" pipe.

Then all you need is a 2 3/4 X pipe in place of the resonator and new mufflers www.advancedautocc.com and look at their X pipe for the LT C4 Vette's and high flo mufflers.

 

[Aurora40]

Then all you need is a 2 3/4 X pipe in place of the resonator and new mufflers www.advancedautocc.com and look at their X pipe for the LT C4 Vette's and high flo mufflers.

The resonance is already right at the limit that I'll tolerate. I suspect an X-pipe won't gain much on a reasonably stock car, and will probably increase the resonance a fair bit.

If I do any more exhaust changes, I'll probably swap over to a Corsa system downstream from some longtubes.